Unit 4: Plant Propagation

Methods of plant propagation (Asexual/Vegetative)

These methods are used to create clones of a parent plant, preserving its desirable characteristics (e.g., a specific fruit flavor or flower color).

Cutting

Cutting: A method where a piece of the parent plant (stem, root, or leaf) is cut off and induced to form its own roots, growing into a new plant.

• Stem Cutting: The most common method. A piece of stem (4-6 inches) is cut, the lower leaves are removed, and it's placed in rooting media. Often, the cut end is dipped in rooting hormone (IBA).
  • Examples: Rose, Bougainvillea, Hibiscus, Coleus, Money Plant.
• Leaf Cutting: A leaf and its petiole (stalk) or just a piece of the leaf blade is used.
  • Examples: Snake Plant (Sansevieria), African Violet, Bryophyllum.

Layering

Layering: A method where roots are induced to form on a stem *while it is still attached to the parent plant*. The new, rooted stem is then detached.

This is a low-risk method because the new plant (layer) is supported by the parent until it has its own roots.

• Simple Layering: A low-lying, flexible branch is bent down to the ground, wounded (by making a small cut), and buried, leaving the tip exposed. Roots form at the wound.
  • Examples: Jasmine, Strawberry.
• Air Layering (or Gootee): Used for woody plants that can't be bent to the ground.
  1. A section of bark (1-inch ring) is removed from a healthy, aerial branch.
  2. The wound is dusted with rooting hormone.
  3. A moist ball of sphagnum moss is wrapped around the wound.
  4. The moss is covered with a plastic sheet, and the ends are tied.
  5. Roots grow into the moss. The branch is then cut below the roots and potted.
  • Examples: Litchi, Guava, Croton, Ficus.

Budding

Budding: A type of grafting where the scion (top part) is reduced to a single bud (with a small piece of bark).

This bud is inserted under the bark of the rootstock (base plant).

• Method (T-Budding):
  1. A T-shaped cut is made in the bark of the rootstock.
  2. A "shield bud" (a bud with a shield-shaped piece of bark) is sliced from the scion plant.
  3. The bud is inserted into the T-cut, and the flaps of bark are closed over it.
  4. The union is wrapped tightly with grafting tape, leaving the bud exposed.
• Examples: This is the primary method for propagating Roses (budding a fancy rose onto a hardy wild rose rootstock) and citrus fruits (Orange, Lemon).

Grafting

Grafting: The art of joining the parts of two different plants (but usually related species) so that they will unite and grow as one.

• Scion: The upper part, which is a small shoot (3-4 buds) from the desired variety (e.g., 'Alphonso' Mango).
• Rootstock (or Stock): The lower part, which forms the root system (e.g., a hardy, native mango seedling).

Principle: The cambium (a thin layer of dividing cells) of the scion must be in close contact with the cambium of the rootstock for the vascular tissues (xylem, phloem) to connect.

Uses: To propagate varieties that don't grow well from cuttings (like Mango, Apple), to get a strong root system, or to change the variety of an existing tree.

Seed structure and seed dormancy

Seed Structure

A mature seed is a "baby plant in a box with its lunch." It consists of three main parts:

1. Embryo: The young, diploid (2n) plant itself. It has:
   • Plumule: The embryonic shoot (becomes the stem and leaves).
   • Radicle: The embryonic root (becomes the primary root).
   • Cotyledon(s): The seed leaves (one in monocots, two in dicots).
2. Endosperm (Food Supply): A starchy or oily tissue that provides food for the embryo. (In some seeds, like beans, the endosperm is absorbed by the cotyledons, which then become the food supply).
3. Seed Coat (Testa): The tough, outer, protective layer that guards the embryo and endosperm from drying out and from pests.

Seed Dormancy

Seed Dormancy: A state in which a viable (living) seed will not germinate even when provided with the ideal conditions (water, oxygen, temperature) for germination.

This is a survival mechanism to ensure the seed only germinates at the *right time* (e.g., in spring, after a fire, after passing through an animal's gut).

Types of Dormancy (and how to break them):

• Physical Dormancy (Seed Coat Dormancy): Caused by a hard, waxy, or impermeable seed coat that prevents water and oxygen from entering.
  • Breaking it (Scarification): Nicking the seed with a file, rubbing with sandpaper, or soaking in acid or hot water to break the seed coat.
• Chemical Dormancy: The seed contains chemical inhibitors (like abscisic acid) that prevent germination.
  • Breaking it (Leaching): Soaking the seed in running water to "wash out" the chemicals.
• Physiological Dormancy: The embryo itself is immature or needs a "trigger."
  • Breaking it (Stratification): Mimicking natural conditions. Cold Stratification involves mixing seeds with moist sand/peat and storing them in a cold (refrigerator) environment for several weeks. This "fools" the seed into thinking it has gone through winter. (Common for apples, cherries).

Concept of plant growth regulators

Plant Growth Regulators (PGRs) / Plant Hormones: Organic chemical messengers produced by the plant (in very low concentrations) that regulate its growth and development from germination to fruiting.

There are five major classes:

1. Auxins (e.g., IAA):
   • Primary Role: Promotes cell elongation, apical dominance (top bud grows, side buds don't).
   • Nursery Use: Synthetic auxins (IBA, NAA) are the main ingredients in "rooting hormone" powder used for cuttings.
2. Gibberellins (e.g., GA3):
   • Primary Role: Promotes stem elongation (causes "bolting"), seed germination, and fruit size.
   • Nursery Use: Used to break seed dormancy and increase fruit size (e.g., grapes).
3. Cytokinins (e.g., Zeatin):
   • Primary Role: Promotes cell division (cytokinesis) and shoot formation.
   • Nursery Use: Used extensively in plant tissue culture (micropropagation) to induce shoot multiplication.
4. Abscisic Acid (ABA):
   • Primary Role: The "stress hormone." Induces dormancy in seeds and buds; causes stomata to close during drought.
5. Ethylene:
   • Primary Role: A gas hormone. Promotes fruit ripening and leaf abscission (falling).
   • Nursery Use: Used commercially to ripen fruits (e.g., bananas) after transport.

Rooting media

This is the substrate used to root cuttings. It is different from potting soil. An ideal rooting medium must be sterile, well-aerated (high oxygen), and moist, but not soggy. It should provide *no* nutrients (this encourages roots to grow in search of food).

Cultivation and harvesting of Potato and Tomato

Potato (Solanum tuberosum)

• Propagation: Asexual (Vegetative). Grown from "seed potatoes," which are small tubers (or pieces of a tuber) that contain at least 1-2 "eyes" (buds).
• Cultivation:
  • Planting: Planted in trenches or furrows in cool weather.
  • "Hilling": As the plant grows, soil is mounded up around the base of the stem. This is essential because the new tubers form on *stolons* (underground stems), and they must be kept in the dark (light turns them green and toxic).
• Harvesting: Harvested when the plant's foliage turns yellow and dies back. Tubers are carefully dug up and "cured" (kept in a cool, dark, humid place) for a week to harden the skin for storage.

Tomato (Solanum lycopersicum)

• Propagation: Sexual (by Seed). Seeds are typically sown indoors (in a nursery) 6-8 weeks before the last frost.
• Cultivation:
  • Transplanting: Young seedlings are transplanted into the garden when the weather is warm.
  • Staking/Training: Tomatoes are vines. They must be given support (a stake, cage, or trellis) to keep the plant and fruit off the ground, which prevents rot and disease.
  • Pruning: (For indeterminate types) "Suckers" (new shoots at the base of leaves) are often pinched off to direct energy into fruit production.
• Harvesting: Fruits are harvested when they are fully colored and firm. They can be harvested at the "breaker" stage (just starting to turn pink) and will ripen fully off the vine.